The number of jet fans required in a tunnel is based on a number of factors, several of which go beyond this page. This page is intented to provide an approximate ball park figure only. The approach used to generate it assumes that the emergency mode is the dominant case in determining what flow is needed to be generated. It further assumes that the critical velocity is approximately 2.7 m/s and the smoke temperature is 250 °C. It is also assumed that the tunnel is unidirectional and has no junctions or open cross passages within it. The calculation is intended to provide high level support to associated disciplines such as civil and electrical, prior to detailed analysis being undertaken.
The calculation considers how the jet fan requirements vary with length of blockage and with external wind forces.
This function will produce plots showing how the tunnel is impacted by vehicle build up and wind.